Songs of the Savannah Sparrow: structure and geographic variation

Abstract

We investigated song structure and the pattern of geographic variation at the syllable and whole song levels in Savannah Sparrow, Passerculus sandwichensis (J.F. Gmelin, 1789). Songs were recorded for 179 males from eight sample populations distributed along a 230 km transect in southwestern Ontario in 2000. Subsequent analysis included measurement and comparison of structural song features and qualitative analysis of syllable and song type similarity among individuals both within and among populations, by reference to a syllable catalogue based on syllable similarity. Song type sharing within samples was not common, while song dissimilarity significantly correlated with distance among samples. Overall south–north trends appeared in two aspects of quantitative and qualitative characteristics, and the geographic differentiation of Savannah Sparrow songs was gradual, increasing with distance. At a higher level of analysis, two song groups, with qualitatively distinct structures (here termed themes), were identified. These themes showed a clinal intergrade, such that one theme was found in all individuals of the four southern samples, while in the four more northerly samples, the second theme's incidence grew progressively from 20% to 80%. However, birds singing the southern theme in populations dominated by the northern theme nevertheless produced songs that show metrical (frequency and duration) characteristics typical of the northern theme songs from their own population. Cluster analysis based on syllable and song type similarity–dissimilarity between populations linked geographically neighboring populations closely, while populations fell into two main clusters: the three most northerly and the five most southerly populations. The same cluster pattern also appeared within each theme type.